advances in the analysis of complex food matrices: species identification in surimi...

RESEARCH ARTICLE

Advances in the analysis of complex food

matrices: Species identification in surimi-

based products using Next Generation

Sequencing technologies

Alice Giusti1☯, Andrea Armani1☯*, Carmen G. Sotelo2*

1 FishLab, Department of Veterinary Sciences, University of Pisa, Pisa, Italy, 2 Instituto de Investigaciones

Marinas (IIM-CSIC), Vigo, Spain

☯ These authors contributed equally to this work.

* [email protected] (AA); [email protected] (CGS)

Abstract

The Next Generation Sequencing (NGS) technologies represent a turning point in the food

inspection field, particularly for species identification in matrices composed of a blend of two

or more species. In this study NGS technologies were applied by testing the usefulness of

the Ion Torrent Personal Genome Machine (PGM) in seafood traceability. Sixteen commer-

cial surimi samples produced both in EU and non-EU countries were analysed. Libraries

were prepared using a universal primer pair able to amplify a short 16SrRNA fragment from

a wide range of fish and cephalopod species. The mislabelling rate of the samples was also

evaluated. Overall, DNA from 13 families, 19 genera and 16 species of fish, and from 3 fami-

lies, 3 genera and 3 species of cephalopods was found with the analysis. Samples produced

in non-EU countries exhibited a higher variability in their composition. 37.5% of the surimi

products were found to be mislabelled. Among them, 25% voluntary declared a species dif-

ferent from those identified and 25% (all produced in non-EU countries) did not report the

presence of molluscs on the label, posing a potential health threat for allergic consumers.

The use of vulnerable species was also proved. Although the protocol should be further opti-

mized, PGM platform proved to be a useful tool for the analysis of complex, highly pro-

cessed products.

Introduction

Present changes in socio-demographic features and people lifestyle, particularly in devel-

oped countries, have radically shifted consumers’ eating habits and their market choices.

With the general increasingly speeding lifestyles and individualisation tendencies, available

time for cooking has in fact reduced, so consumers normally prefer “time saving” products

as well as affordable prices. Ready-to-eat products, which do not require a further heating or

processing step before consumption, have increasingly appeal consents due to their cheap-

ness, storage easiness and attractive appearance [1]. Among products of animal origin, many

PLOS ONE | https://doi.org/10.1371/journal.pone.0185586 October 2, 2017 1 / 18

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OPENACCESS

Citation: Giusti A, Armani A, Sotelo CG (2017)

Advances in the analysis of complex food matrices:

Species identification in surimi-based products

using Next Generation Sequencing technologies.

PLoS ONE 12(10): e0185586. https://doi.org/

10.1371/journal.pone.0185586

Editor: Massimo Labra, Universita degli Studi di

Milano-Bicocca, ITALY

Received: June 14, 2017

Accepted: September 17, 2017

Published: October 2, 2017

Copyright: © 2017 Giusti et al. This is an open

access article distributed under the terms of the

Creative Commons Attribution License, which

permits unrestricted use, distribution, and

reproduction in any medium, provided the original

author and source are credited.

Data Availability Statement: All relevant data are

within the paper.

Funding: LABELFISH. ATLANTIC NETWORK ON

GENETIC CONTROL OF FISH AND SEAFOOD

LABELLING AND TRACEABILITY, funded by

Atlantic Area Programme (INTERREG IVB) 2011-1/

163 REF 1070134; URL: http://www.atlanticarea.

eu/. The funders had no role in study design, data

collection and analysis, decision to publish, or

preparation of the manuscript.

https://doi.org/10.1371/journal.pone.0185586

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http://creativecommons.org/licenses/by/4.0/

http://www.atlanticarea.eu/

http://www.atlanticarea.eu/

processed foods can be included under the definition of ready-to-eat food, such as ham, sau-

sages, dairy products (milk, cheese, spreads), smoked fish, prepared salads, nuggets and

others.

Surimi is a stabilized myofibrillar protein compound obtained from mechanically deboned

fish flesh that is repeatedly washed with water and blended with cryoprotectants [2]. This fish

paste represents an intermediate product used in the preparation of a variety of ready-to-eat

seafood commodities, called surimi-based products (SBPs), marketed in different forms such

as sticks, slices, crumbs, lobster tails-like, etc. [3]. SBPs, originally produced, marketed and

consumed in Asian countries, are increasingly appreciated worldwide, especially in North

America and Europe [4]. To date, they are in fact commonly produced also by Western food

processing industries. Initially, Alaska pollock (Gadus chalcogrammus) was the main species

used for surimi production. Then, due to its overexploitation, numerous previously underuti-

lized fish species have started to be used [2, 5–7]. Cephalopods, particularly squids, are also

often used in surimi manufacture [2,4], mainly thanks to the gelation properties of their pro-

teins or as flavouring ingredients [8]. Therefore, surimi represents a multispecies seafood

product, as its production can imply the use of an extremely wide range of species [2,5].

According to the current EU law on food labelling, it is not mandatory to provide the commer-

cial and/or scientific name of the seafood species present in SBPs [9–11], although some

brands report it voluntarily (author’s note). However, the presence of ingredients potentially

causing allergies, listed in the Regulation (EU) No 1169/2011 (including “fish” and “molluscs”)

must be declared. As they never represent the major ingredient of SBPs, the use of cephalopods

may be undeclared, causing a potential hazard for allergic consumers.

Since surimi is a highly processed product, the use of morphological characters to identify

which species have been used is impossible. Thus, species identification through DNA analy-

sis is useful to verify the information reported on the label (if the species is declared) and to

detect the eventual presence of undeclared allergenic ingredients such as cephalopods. More-

over, it allows to promote the sustainable environmental management, particularly if overex-

ploited and/or endangered fish species are used. SBPs were actually proved to be particularly

involved in mislabelling cases, regardless of their origin [5–7]. However, few studies assessing

the composition of these types of products have been conducted since now, as the possibility

to detect species within products containing a mixture of species goes beyond the capability

of the analytical techniques routinely applied in food control. Available studies applied

DNA-based techniques involving a classical Sanger-based DNA sequencing phase, such as

FINS [7] as well as DNA-Barcoding [6]. However, Sanger-based DNA sequencing alone has

often been considered a not feasible approach for a complete description of species composi-

tion in mixed food analysis. Galal-Khallaf et al. [5] have recently highlighted the necessity to

appeal to more suitable techniques for these products, combining the classical direct

sequencing of PCR products with a PCR cloning technique with subsequent plasmid

sequencing. PCR cloning, even though effective, is a rather laborious and time-consuming

approach to be routinely used in laboratories. In this regard, a metagenomics approach,

using High Throughput Sequencing technologies, commonly known as Next Generation

Sequencing (NGS), represents a useful alternative to PCR cloning to identify species in highly

processed multispecies products. This technique is faster and even more informative than

cloning, since it can detect also low-represented species in mixtures [12]. Because of that,

NGS results attractive for food inspection research field, even though they cannot yet be con-

sidered enough mature to be applied as routine method. More studies aimed at improving its

accuracy as well as correcting their error sources are needed [13]. Preliminary studies were

performed on artificial mixtures of meat species to verify the method’s robustness [14,15]

and the NGS have been practically applied to commercial products in the work of Muñoz-

Species identification in surimi-based products using Next Generation Sequencing technologies


Competing interests: The authors have declared

that no competing interests exist.


Colmenero et al. [16], that detected the animal species contained in candies, that were

selected as a model of highly processed foods. As regards seafood, to the best of our knowl-

edge, NGS technologies have been applied in commercial fish cakes [17], as well as in highly

processed cod products [18]. In addition, Kappel et al. [19] tested their effectiveness in dis-

criminating tuna species within experimental mixtures. Given the scarce available literature,

more studies focused in optimizing NGS protocols and testing the potentiality of these tech-

nologies in seafood analysis are undoubtedly required. In fact, although the still quite high

costs, NGS prices are progressively dropping during the years, so that, in a near future, these

techniques could be routinely applied to this research field.

A preliminary study, conducted with the aim to help in the preparation phase of NGS

libraries, showed the ability of the primer’s pair developed by Chapela et al. [20] to amplify a

short fragment of mitochondrial 16S ribosomal gene (16SrRNA) in many fish and cephalopod

species used in surimi production [21]. In the present study, we used for the first time the Ion

Torrent Personal Genome Machine (PGM) to apply a metabarcoding approach to the analysis

of the composition of some surimi-based products (SBPs) purchased on international market.

The primer’s pair of Chapela et al. [20] was used for amplifying the DNA fragment to be

turned into standard libraries. This study aimed at providing an analytical starting point to

better approach such new techniques for their future application to a wider range of multispe-

cies seafood products.

Materials and methods

Samples collection and DNA extraction

Sixteen SBPs were collected (Table 1). Among them, fourteen were purchased from Spanish

and Italian grocery stores and two were collected from imports coming from third countries

by the staff of the Border Inspection Post (BIP) of Leghorn (Italy). All the samples were stored

at -20˚C before DNA extraction. Total DNA was extracted from all the SBPs with the protocol

proposed by Armani et al. [22], starting from 100 mg of tissue, and quantified using a Qubit™3.0 Fluorometer (Thermo Fisher Scientific).

DNA amplification and purification

The DNA samples were amplified with the primer pairs 16sf-var 5´-CAAATTACGCTGTTATCCCTATGG-3´ and 16sr-var 5´-GACGAGAAGACCCTAATGAGCTTT-3´designed by Cha-

pela et al. [20] using illustra™ puReTaq Ready-To-Go™ PCR Beads (GE Healthcare). For each

tube containing the bead, 2 μl of 200 nM of each primer, 1 μl of 50 ng of template DNA and

nuclease-free water (Life Technologies) were added, for a final reaction volume of 25 μl. DNA

was amplified on an Applied Biosystems Veriti™ 96 well Thermal Cycler (Thermo Fisher Scien-

tific) with the following cycling program: denaturation at 94˚C for 3 min; 35 cycles at 94˚C for

40 s, 60˚C for 40 s, and 72˚C for 40 s; final extension at 72˚C for 7 min. 5 μL of each PCR prod-

uct was checked by electrophoresis on a 2% agarose gel and the presence of fragments of the

expected length was assessed by a comparison with the standard marker O’GeneRuler DNA

Ladder (Thermo Fisher Scientific). Double-stranded PCR products were purified with Agen-

court1 AMPure1 XP Kit for DNA purification (Beckman Coulter, Beverly, Massachusetts,

USA) on a DynaMag™-2 magnet magnetic rack (Thermo Fisher Scientific) following the proce-

dure proposed by the manufacturer. Purified PCR products were quantified using a Qubit™ 3.0

Fluorometer (Thermo Fisher Scientific).




Table 1. Commercial SBPs analysed in this study.

Sample

(IC)

Collection

site

Producer

country

Commercial denomination/

product description

Ingredients Declared species

SUR-1 Big

distribution

(Spain)

Spain Surimi product—Sticks Surimi (fish), water, sunflower oil, cephalopod (mollusc), starch and modified

starch, salt, egg albumin, vegetable protein, flavor (containing shellfish), flavor

enhancer (monosodium glutamate), sugar, natural dyes (cochineal and

paprika extract)

ND

SUR-2 Big

distribution

(Spain)

Germany Cooled surimi product Surimi (Fish), water, cephalopod (mollusc), sunflower oil, starch modified,

wheat flour, soy protein, sea salt, vegetable protein, egg albumen, flavor

(contains crustaceans) and cephalopod ink (mollusc)

ND

SUR-3 Big

distribution

(Spain)

Spain Grated surimi product Surimi (fish), water, sunflower oil, cephalopod (mollusc), starch and modified

starch, salt, egg albumin, vegetable protein, flavor (containing crustacea),

flavor enhancer (monosodium glutamate), sugar, natural colorings (cochineal

and paprika extract)

ND

SUR-4 Big

distribution

(Spain)

Spain Sticks Surimi 47% [fish and cephalopods (molluscs)], water, corn starch, modified

starches (gluten), sunflower oil, aroma and crab extract [crustaceans, soy

flavor enhancer (monosodium glutamate, E635)], salt, egg, vegetable protein

(gluten), coloring (paprika extract)

ND

SUR-5 Big

distribution

(Spain)

Spain Surimi slices Surimi 49% (fish), water, rice starch, sunflower oil, crab aroma and extract

[crustaceans, molluscs, soy flavor enhancers (monosodium glutamate, E

635)], egg albumin, protein vegetable (gluten), modified starch (gluten), salt,

sugar, preservative (potassium sorbate, coloring (paprika extract)

ND

SUR-6 Big

distribution

(Spain)

France Surimi duo–lobster flavour 38% Surimi (Fish pulp 92% -Micromesistius poutassou, sugar, trehalose,

stabilizers: sorbitols, E450, E451), water, wheat starch (contains gluten),

rehydrated egg powder, canola oil, salt, flavorings (contains gluten, fish and

shellfish), sugar, flavor enhancer: monosodium glutamate; coloring: paprika

extract

Micromesistius

poutassou

SUR-7 Big

distribution

(Spain)

Spain Surimi sticks–seafood

flavour

Surimi (Fish), water, starch and modified starch, cephalopod (mollusc),

sunflower oil, egg whites, vegetable protein, salt, flavor enhancer (E621),

flavor (contains crustaceans), sugar, coloring (E120 and E160c)

ND

SUR-8 Big

distribution

(Spain)

Spain Surimi sticks Surimi (Fish), water, sunflower oil, cephalopod (mollusc), starch and modified

starch, crab extracto (crustacean), salt, egg whites, vegetable protein, flavor

enhancer (monosodium glutamate), white wine extract, sugar, natural

colorings (cochineal and paprika extract)

ND

SUR-9 Big

distribution

(Spain)

Spain Processed seafood product

with garlic

Fish protein, water, sunflower oil, wheat flour, cephalopod (mollusc), olive

oil, salt, soy protein, vegetable protein, milk protein, egg white, aromas, flavor

enhancer (glutamate monosodium), stabilizer (xanthan gum), acidity (lactic

acid), cephalopod ink, (mollusc), natural extracts (garlic and chilli). May contain

traces of crustacea.

ND

SUR-10 Big

distribution

(Spain)

Spain Surimi sticks–seafood

flavour

Surimi (Fish), water, sunflower oil, cephalopod (mollusc), starch and modified

starch, salt, egg white, vegetable protein, flavorings (contain crustaceans),

flavor enhancer (monosodium glutamate), sugar, natural colorings (cochineal

and paprika extract)

ND

SUR-11 Big

distribution

(Spain)

Spain Surimi product Surimi (Fish), water, sunflower oil, wheat flour, cephalopod (mollusc), salt,

soy protein, vegetable protein, milk protein, egg albumen, aromas, flavor

enhancer (E621), stabilizer (E415), acidity regulator (E270), cephalopod ink

(mollusc)

ND

SUR-12 Big

distribution

(Spain)

Spain Surimi in brine–crab flavour White fish, squid, sugar, sorbitol (E420), polyphosphates (E452), water,

potato starch, crabmeat (4%), vegetable oil, egg, salt, flavoring crab, flavor

enhancers (E621, E635), stabilizer (carrageenan), colorants (E171, E120,

E160c). Pickle ingredients: water, salt, citric acid, trisodium citrate, sodium

benzoate, tartaric acid

ND

SUR-13 Small

retailers

(Italy)

Korea Lobster tails imitation Alaska pollock (55,78%), water, wheat starch, egg white, lobster extract, rice

alcohol, salt, sugar, soybean oil, natural coloring: paprika

Gadus

chalcogrammus

SUR-14 Small

retailers

(Italy)

Lithuania Surimi sticks–crab flavour Surimi (38%), water, starch, egg white, rapeseed oil, soy proteins, salt, sugar,

tapioca starch and acetylated potato, crab aroma, whole egg, E621, E631,

E635, E160, E120

ND

SUR-15 BIP

(Italy)

China Surimi sticks–crab flavour Surimi 44%, wheat starch, soybean oil, crab flavor, crab extract, egg white,

potato flour

Nemipterus sp.

SUR-16 BIP

(Italy)

Thailand Fish-shaped crab meat

imitation

Surimi 35%, palm oil, modified tapioca starch, egg white, soy protein, crab

extract.

Nemipterus sp.

IC: Internal code BIP: Border Inspection Post

https://doi.org/10.1371/journal.pone.0185586.t001





Preparation of barcoded libraries

A specific barcoded library was prepared for the amplicon obtained from each SBPs using the

Ion Plus Fragment Library Kit (Thermo Fisher Scientific) (IPFL kit), that allowed amplicons’

end-repair and ligation to Ion-compatible adapters.

Amplicons end-repair and purification. 20 ng of each amplified sample were diluted in a

total volume of 79 μl of nuclease-free water (Life Technologies). Amplicons’ end-repair was

done by adding 20 μl of 5X End Repair Buffer and 1 μl of End Repair Enzyme (both provided

by the IPFL kit) and incubating the reaction at room temperature for 20 minutes. The samples

were then purified with Agencourt1 AMPure1 XP Kit for DNA purification (Beckman Coul-

ter, Beverly, Massachusetts, USA) on a DynaMag™-2 magnet magnetic rack (Thermo Fisher

Scientific) following the procedure proposed by the manufacturer.

Adaptors ligation, nick reparation and purification of the amplicons. Adaptors pro-

vided in the Ion Xpress™ Barcode Adapters 1–16 (Thermo Fisher Scientific) were used. The

same Ion Xpress™ P1 Adapter was ligated to the amplicons obtained from all the SBPs samples

whereas a unique Ion Xpress™ Barcode Adapter for each sample was used. Adaptors ligation

and nick repair phases were done in a final reaction volume of 100 μl, containing: 25 μl of end-

repaired and purified amplicon with 10 μl of 10X Ligase Buffer, 2 μl of dNTP Mix, 2 μl of DNA

ligase and 8 μl of Nick Repair Polymerase (all provided by the IPFL kit), 2 μl of Ion P1 Adapter,

2 μl of Ion Xpress™ Barcode Adapter, 49 μl of nuclease-free water (Life Technologies). Each

reaction mix tube was run on an Applied Biosystems Veriti™ 96 well Thermal Cycler (Thermo

Fisher Scientific) with the program proposed by the IPFL kit manufacturer. The samples were

then purified with Agencourt1 AMPure1 XP Kit for DNA purification (Beckman Coulter,

Beverly, Massachusetts, USA) on a DynaMag™-2 magnet magnetic rack (Thermo Fisher Scien-

tific) following the procedure proposed by the manufacturer. Purified products were quanti-

fied by an Agilent 2100 Bioanalyzer (Agilent Genomics).

Libraries amplification and quantification. Libraries were amplified on an Applied

Biosystems Veriti™ 96 well Thermal Cycler (Thermo Fisher Scientific) in a total reaction vol-

ume of 130 μl, containing 100 μl of Platinum1PCR SuperMix High Fidelity, 5 μl of Library

Amplification Primer Mix (both provided by the IPFL kit) and 25 μl of unamplified library.

The cycling program suggested on the IPFL kit protocol was applied. Amplified libraries were

purified with Agencourt1 AMPure1 XP Kit for DNA purification (Beckman Coulter, Beverly,

Massachusetts, USA) on a DynaMag™-2 magnet magnetic rack (Thermo Fisher Scientific)

following the procedure proposed by the manufacturer. Agilent 2100 Bioanalyzer (Agilent

Genomics) was used to determine the molar concentration of each barcoded library. Three

equimolar pools of barcoded libraries were prepared: barcoded libraries from SUR-1 to SUR-6

(Pool 1), from SUR-7 to SUR-12 (Pool 2) and from SUR-13 to SUR-16 (Pool 3) were pooled

together. The three pools were quantified on Agilent 2100 Bioanalyzer (Agilent Genomics) or

Library TaqManTM Quantitation Kit (Thermo Fisher Scientific) following the procedure pro-

posed by the manufacturer, and then diluted as proposed by the Ion PGM™ Hi-Q™ Chef Kit

(Thermo Fisher Scientific).

Massive DNA clonal parallel amplification and sequencing by synthesis

Ion Sphere™ Particles (ISP) preparation and chips loading. The Ion PGM™ Hi-Q™ Chef

Kit (Thermo Fisher Scientific) was utilized to prepare template-positive Ion Sphere™ Particles

(ISP) and to load three Ion 314™ v2 BC sequencing chips (Chip 1 for Pool 1, Chip 2 for Pool 2

and Chip 3 for Pool 3) on Ion Chef™ System (Thermo Fisher Scientific) following the manufac-

turer protocol.




Sequencing by synthesis. The three chips sequencing was done on an Ion PGM™System

(Thermo Fisher Scientific) using the Ion PGM™ Hi-Q™ Sequencing Kit (Thermo Fisher Scien-

tific) according to the manufacturer protocol. Reads obtained from the three sequencing chips

were processed by the software Torrent Suite™ version 5.0 (Thermo Fisher Scientific).

Bioinformatics analysis

Data quality assessment. Each sequencing chip was primarily overall evaluated with the

Torrent Suite™ version 5.0 (Thermo Fisher Scientific) software on the basis of the final number

of usable reads (overall quality assessment) and of the reads length. Regarding the overall qual-

ity, we considered as acceptable a ISP loading higher than 70%, jointly with a polyclonal

amount lower than 20% and a final percentage of usable library higher than 80% (weak pres-

ence of low quality reads). The reads length was considered as cornerstone of a good sequenc-

ing outcome if the distribution of the major part of the reads (expressed as a graphic peak)

corresponded to the length of the target amplicon. Then, the FASTQ files for each barcoded

sample (that contain the raw sequences and their quality values) were downloaded from the

software and analysed through the program FastQC High Throughput Sequence QC Report

version 0.11.5 (www.bioinformatics.babraham.ac.uk/projects/). We put attention to the total

number of the raw reads and to their length (both provided by the FastQC program), that

might be long enough to include the target amplicon.

Reads taxonomic assignment and analysis of frequencies. The raw FASTQ files were

sent to Era7 Bioinformatics (Cambridge MA, USA) for obtaining their taxonomic profile. In

details, according to the final report providing from Era7, the sequences were filtered on the

basis of a minimum length of 100 bp and a maximum of ~300 bp (to contain the target ampli-

con). The sequences were also filtered on the basis of their quality in order to ensure a highly

supported taxonomic assignment. Filtered reads were then assigned to a taxonomic tree node

based on sequence similarity to 16SrRNA genes included in Era7 internal database, built with

16S sequences extracted from from RNAcentral database (http://rnacentral.org/). RNAcentral

database includes rRNAs from a wide set of important databases as SILVA, GreenGenes, RDP,

RefSeq and ENA. The NCBI taxonomy was used and for taxonomic assignment the MG7

method, that is based on a BLAST comparison of each read against the 16S ribosomal RNA

database, was applied. Samples’ species identification was based on the BLAST results. In par-

ticular, taxonomic assignment was done using 2 different algorithms: (i) Best BLAST Hit

(BBH) assignment, obtained by the BLASTN of each read against the internal 16S database

(each read was assigned to the taxon corresponding to the Best Blast Hit over a threshold of

similarity) and (ii) Lowest Common Ancestor (LCA) assignment, where each read was

assigned to the most probable taxon where it could come from. The frequencies for each tax-

onomy node were also assessed. Phylum, Family, Genus and species distribution in all the

samples was assessed. The frequencies were expressed in % with respect to the total merged

reads of each sample. Moreover, BBH assignments were used to calculate the diversity index

for each sample. In particular, Simpson’s diversity index was applied.

SBPs mislabelling assessing

A preliminary analysis of the information reported on the label was performed in the light of

the current European legislation [9,10] and coupled with the analytical results to evaluate the

mislabelling degree of the SBPs. In particular, we used the following criteria to consider one

case of mislabelling:: (A) labels did not report the precise term “fish” among the ingredients

(B) labels did not report the precise term “molluscs” among the ingredients (whereas

declared); (C) among the labels voluntarily reporting the scientific name, those in which the



http://www.bioinformatics.babraham.ac.uk/projects/

http://rnacentral.org/


declared species did not correspond to the ones retrieved by the analysis; (D) labels did not

declare the presence of molluscs but the analysis proved the presence of species belonging to

this Phylum.

Results and discussion

Samples collection

The traditional SBPs trade, based on Chinese exports to Europe, has recently slowed down

since Europe has increased its own SBPs manufacturing activity. Spain plays an important role

in the European surimi sector, being one of the most important producer and consumer of

SBPs, jointly with France [4]. Apart from the two SBPs collected at the BIP (SUR-15 and SUR-

16), produced in China and Thailand respectively, the fourteen samples directly purchased in

this study in Spain and Italy were produced in European countries, except for one (SUR-13)

which was produced in Korea and purchased at an Italian small retailer. Twelve samples

reported on the label the presence of “fish” in the list of ingredients, sometimes adding an

adjective or a noun such as “white fish”, “fish pulp” or “fish protein”. The scientific name of

the utilized species was reported in four samples and corresponded to Micromesistius poutas-sou (SUR-6), Gadus chalcogrammus (SUR-13) and Nemipterus spp. (SUR-15 and SUR-16).

Ten samples reported the presence of “molluscs”, either among the main ingredients or as

aroma/extract, or both. The percentage of surimi paste was also reported in seven samples. In

all the SBPs miscellaneous ingredients were also listed, such as wheat starch, potato, salt, soy-

bean oil and sugar (Table 1).

Primers selection

Since the maximum target read length of Ion PGM sequencing system is 400 bp, the amplicon

selected as target must be shorter. The primer pair used in this study, already tested in a previ-

ous study [20], was proved able to amplify a fragment of ~250–260 bp (depending on the

species) from fish as well as a fragment of ~190–200 bp (depending on the species) from ceph-

alopod species. Therefore, it can be successfully used for the amplification of extremely pro-

cessed products such as surimi, where a high degree of DNA degradation is known [7]. In

addition, although they have been originally designed to amplify only cephalopod species [20],

a recent study [21] has shown their capability in amplifying DNA from more than eighty spe-

cies of fish and cephalopods.

High Throughput Sequencing and data analysis

Overall quality control of the reads. Modern high throughput sequencers can generate

tens of millions of sequences in a single run. Therefore, preliminary suitable quality control

checks of the raw data are required before approaching subsequent analysis.

Overall, 674.983 raw sequences (78% of total analysed) and 674.826 raw sequences (73% of

total analysed) were obtained from Chip 1 and Chip 2, respectively. The sequences were con-

sidered as “usable” since they respected the threshold values established in materials and meth-

ods section (see paragraph “Data quality assessment”). Moreover, the global distribution peak

of the reads corresponded to the length of the target amplicon. Chip 3 was less performant as

the 668.081 raw sequences evaluated as usable represented only 58% of all the sequences ana-

lysed. Moreover, the polyclonal percentage (32%) was higher than the threshold value. How-

ever, since the final usable library was good (85%), few low-quality sequences were present

(14%) and the read lengths corresponded to the expected length of the target amplicons, these

data were considered as usable for subsequent bioinformatics analysis.




FastQC analysis. Comparison between FastQC analysis of raw and filtered reads per each

sample was reported in Table 2. Raw reads considered suitable in term of quality by Torrent

Suite™ version 5.0 software (Thermo Fisher Scientific) ranked between 2431 and 259531 and

their length ranked between a minimum of 25 bp to a maximum of 354 bp. Filtration phase

reduced the number of reads by selecting a minimum length of 100 bp, while the maximum

length was between 262 bp and 321 bp. This step allowed to preserve the two target amplicons

(250–260 bp for fish and 190–200 for cephalopods) and easing the taxonomic analysis by

removing too short fragments that could have given uncertain results. After the filtration step,

the total number of usable reads was not much lower than that of raw reads, with a preserva-

tion ranging from 75.3% to 88.2%, in the case of Chip 1 and Chip 2. On the contrary, final

usable reads of Chip 3 were lower, with a preservation ranging from 37.8% to 51.1%, confirm-

ing the worse outputs obtained in the previous overall reads analysis.

Reads taxonomic assignment, analysis of frequencies and samples diversity index.

Phylum distribution (cumulative % LCA) in the samples is shown in Fig 1. Even though, as

Table 2. Comparison between FastQC analysis of raw and filtered reads obtained from each sample.

Sample Raw reads Filtered reads Preserved reads (%)

Number Length (bp) Number Length (bp)

SUR-1 21597 25–333 19056 100–313 88.2

SUR-2 79174 25–332 68051 100–299 86

SUR-3 69682 25–292 59820 100–278 85.8

SUR-4 154476 25–325 126906 100–310 82.2

SUR-5 184772 25–346 155618 100–294 84.2

SUR-6 106418 25–354 91054 100–311 85.6

SUR-7 68584 25–312 53647 100–290 78.2

SUR-8 2431 25–262 1831 100–262 75.3

SUR-9 72162 25–318 59494 100–292 82.4

SUR-10 82491 25–321 64473 100–321 78.1

SUR-11 259531 25–319 207017 100–302 79.8

SUR-12 115899 25–339 96113 100–299 82.9

SUR-13 54134 25–307 23853 100–277 44.1

SUR-14 54572 25–297 28104 100–292 51.5

SUR-15 38203 25–296 17976 100–293 47.1

SUR-16 57674 25–316 21815 100–300 37.8


Fig 1. Phylum distribution (cumulative % LCA) in each analysed SBP.

https://doi.org/10.1371/journal.pone.0185586.g001






predictable, the Phylum Cordata was the most represented in all the SBPs (always greater than

75% and in 75% of the samples greater than 90%), the Phylum Mollusca was also found in

100% of the samples, in different percentages. In particular, 25% of the samples contained less

than 1% of molluscs’ DNA, 50% between 1% and 10%, 12.5% between 10% and 20%, and

12.5% more than 20%. Regardless of the quantities of DNA found in the samples, these results

substantially confirmed that the use of molluscs in surimi products is very common in the sea-

food industry. Literature reported in fact the high resistance to freeze-induced denaturation

and to proteolytic attack of cephalopods myofibrillar proteins [2], as well as the fact that even a

small amount of these proteins considerably improves the texture of a gel product, making it

more elastic and with a greater cohesiveness [23].

Results of the family, genus and species distribution in all the samples were reported in

Table 3. Given the great amount of assigned reads, only taxonomical entities present in the

samples in amount >1% were reported. Overall, DNA from 13 families, 19 genera and 16 spe-

cies of fish, and from 3 families, 3 genera and 3 species of cephalopods was found in SBPs. Figs

2 and 3 depict the distribution of families and genera in the samples. Regarding fish, although

some differences in composition between EU and Asian SBPs subsisted, DNA belonging to

the Gadidae family was found in 100% of the samples. The percentage was rather high in most

of the cases, exceeding 90% in 50% of the samples, ranging from 70 to 90% in 31.2% of the

samples and from 40 to 70% in 12.5% of the samples. Gadidae were poorly represented (<2%)

only in one Asian sample (SUR-13). DNA from Gadus genus was found in 100% of the sam-

ples, with the species Gadus chalcogrammus identified in 93.75% of the samples, whereas DNA

from Gadus morhua was detected in two samples. Also, DNA from Arctogadus genus/Arctoga-dus glacialis species, Melanogrammus genus/Melanogrammus aeglefinus species and Merlan-gius genus/Merlangius merlangus species was detected in 6.25%, 43.75% and 6.25% of the

samples, respectively. DNA belonging to Merluccidae family was found in 25% of the samples,

in variable percentages (from’ 1% to over 36%). and onlyMerluccius genus/Merluccius mer-luccius species was present in that samples. 12.3% of DNA from Nemipteridae family/Nemip-terus genus (species identification was not reached) was found only in SUR-4. Variable

percentage of DNA belonging to Carangidae (Trachurus spp.), Synodontidae (Saurida undos-quamis), Clupeidae (Dorosoma petenense and Ethmalosa fimabriata), Percidae (Sander spp.),

Engraulidae (Coilia grayii) Caesionidae (Pterocaesio tile), Siganidae (Siganus spp.), Lutjanidae

(Lutjanus bengalensis and Lutjanus rivulatus) and also to freshwater families such as Osphoro-

nemidae (Trichopodus leeri) and Cichlidae (Etroplus maculatus and Paretroplus maculatus) was

found in some samples. It is important to underline that in all the samples a variable percent-

age of DNA not identifiable at species level, but only at family or genus level, was present

(Table 3). These results substantially confirmed those already reported in literature. In fact, the

most part of the fish species found in the samples were those commonly used for surimi pro-

duction or sometimes reported in studies aimed at identifying species in such type of products.

Exceptions were represented by the non-EU samples SUR-13, SUR-15 and SUR-16 and by the

EU sample SUR-7, in which unconventionally species, never used until now, were found.

Regarding molluscs, it is primarily important to reiterate the fact that, although DNA from

molluscs was detected in all the samples, taxonomical assignment was reported in Table 3 only

if its presence was higher than 1%. DNA from Ommastrephidae family/Todarodes genus/

Todarodes pacificus species was found in 75% of the samples. DNA belonging to Loliginidae

family/Doryteuthis genus/ Doryteuthis opalescens species and Architeuthidae family/Archi-teuthis genus/Architeuthis dux species was also found in one sample (SUR-13). Differently

from fish, the molluscs species found in the analysed samples did not correspond to those

listed as the most used in surimi manufacture. In fact, the species T. pacificus, which was the




Table 3. Family, genus and species distribution in all the samples with relative percentages.

Sample Family Genus Species

SUR-1 Cordata Gadidae 91.2% Gadus 54.5% Merlangius 1.1% Gadus chalcogrammus 54.2% Gadidae 34.9%

Merlangius merlangus 1.1%

Mollusca Ommastrephidae 8.6% Todarodes 8.6% Todarodes pacificus 8.6%

SUR-2 Cordata Gadidae 85.1%Merluccidae 1.5% Gadus 57.1%Merluccius 1.1% Gadus chalcogrammus 56.9% Gadidae 27.0%

Merluccius merluccius 1.5%


SUR-3 Cordata Gadidae 98.2% Gadus 63.5% Gadus chalcogrammus 63.20%Gadidae 33.6%


SUR-4 Cordata Gadidae 75.0% Nemipteridae 12.3% Gadus 49.1% Nemipterus 12.3% Gadus morhua 25.5% Gadidae 25.0% Gadus

chalcogrammus 19.7% Nemipterus spp. 12.3%

Gadus spp. 3.8%


SUR-5 Cordata Gadidae 42.9% Merluccidae 36.3% Merluccius 36.3% Gadus 28.9% Merluccius merluccius 36.3% Gadus chalcogrammus

28.8% Gadidae 13.5%


SUR-6 Cordata Gadidae 99.3% Gadus 51.6% Arctogadus 2.7%

Melanogrammus 1.3%

Gadidae 43.2% Gadus morhua 41.9% Gadus spp.

9.1% Arctogadus glacialis 2.7% Melanogrammus

aeglefinus 1.3%

SUR-7 Cordata Gadidae 67.2% Cichlidae 10.7% Merluccidae

8.5% Percidae 1.3%

Gadus 36.9% Merluccius 8.6% Paretroplus

5.1% Etroplus 4.7% Sander 1.3%

Melanogrammus 1.1%

Gadus chalcogrammus 36.7% Gadidae 28.9%

Merluccius merluccius 8.5% Paretroplus maculatus

5.1% Etroplus maculatus 4.7% Sander spp. 1.3%

Melanogrammus aeglefinus 1.1%


SUR-8 Cordata Gadidae 95.3% Gadus 53.1% Melanogrammus 1.5% Gadus chalcogrammus 52.4% Gadidae 39.7%










SUR-12 Cordata Gadidae 89.1% Merluccidae 2.7% Gadus 58.5% Merluccius 2.7%

Melanogrammus 1.4%


Merluccius merluccius 2.7% Melanogrammus

aeglefinus 1.4%


SUR-13 Cordata Synodontidae 19.9% Clupeidae 18.4% Percidae

9.1% Lutjanidae 7.1% Carangidae 4.6%

Osphoronemidae 2.7% Gadidae 1.6%

Scombridae 1.2% Engraulidae 1.1%

Saurida 19.9% Ethmalosa 13.6% Sander

8.8% Lutjanus 6.9% Trachurus 4.2%

Dorosoma 3.8% Trichopodus 2.6% Gadus

1.3% Coilia 1.1%

Saurida undosquamis 19.9% Ethmalosa fimbriata

13.6% Sander spp. 8.8% Percomorphaceae 4.8%

Trachurus spp. 4.2% Lutjanus spp. 4.1% Dorosoma

petenense 3.6% Lutjanus rivulatus 2.3% Trichopodus

spp. 1.3% Gadus chalcogrammus 1.3% Trichopodus

leeri 1.2% Coilia grayii 1.0%

Mollusca Architeuthidae 13.9% Loliginidae 6.9%

Ommastrephidae 1.5%

Architeuthis 13.9% Doryteuthis 6.9%

Todarodes 1.5%

Architeuthis dux 13.9% Doryteuthis opalescens 6.9%

Todarodes pacificus 1.5%

SUR-14 Cordata Gadidae 96.4% Gadus 66.9% Gadus chalcogrammus 66.3% Gadidae 28.2%


SUR-15 Cordata Gadidae 85.8% Lutjanidae 3.5% Percidae 3.4% Gadus 57.7% Sander 2.9% Lutjanus 2.5%

Pterocaesio 1.0%


Sander spp. 2.9% Gadus morhua 1.8% Lutjanus

bengalensis 1.1% Lutjanus rivulatus 1.0%

Pterocaesio tile 1.0%


SUR-16 Cordata Gadidae 80.8% Lutjanidae 8.5% Gadus 51.7% Lutjanus 8.3% Siganus 1.8%

Scomber 1.0%


Lutjanus bengalensis 5.8% Gadus morhua 2.4%

Lutjanus rivulatus 2.1% Siganus spp. 1.8%






most representative in this study, was very rarely reported, while the species A. dux was never

reported at all.

Diversity index was calculated to reflect how many different species there were in each sam-

ple. The obtained values of the Simpson’s diversity index (D Index) were reported and graphi-

cally illustrated in Fig 4. The most part of the samples (43.7%) presented a diversity index

between 0.5 and 0.6. The highest diversity index values (�0.8) were reached by the samples

SUR-4 and SUR-13, whereas the lowest (�0.5) by the samples SUR-3 and SUR-14. The diver-

sity index value did not seem to be directly correlated to the country origin of the sample.

SBPs’ label information: Mislabelling assessment

Mislabelling evaluation results were reported in Table 4. Overall, 37.5% of the SBPs’ were

found as mislabelled. Non-compliance involved 100% of SBPs produced in non-EU countries

and 23.1% of SBPs produced in EU countries. The final mislabelling percentage was: (i) 25% of

the samples did not report the exact term “fish”, but were simply labelled as “surimi” or, in the

Fig 2. Families presence and distribution in each sample. (E): SBP produced in Europe: (A): SBP produced in Asia.






case of SUR-13, only declared the commercial denomination of the species (“Alaska pollock”)

jointly with the scientific name (Gadus chalcogrammus). The use of the term “surimi” alone, as

well as the species declaration alone, are both non-compliant with the current Regulation since

it could be unclear, for the average consumer, that surimi is often produced from fish and mol-

luscs. The presence of allergens such as fish or mollusc should be clearly indicated in the label

since allergic consumers might not be aware of what they are really buying; (ii) 12,5% of the

samples reported an altered form of the term “molluscs”, indicating “shellfish” or “squid”,

which could be equally misleading and unclear for consumers; (iii) 25% of the samples volun-

tary declared a species that actually not corresponded to that found through the DNA analysis.

In SUR-6, where the presence of Micromesistius poutassou was declared, no DNA of this spe-

cies was found. On the contrary, the most part of DNA found in this sample belonged to the

species Gadus morhua or, in lower amount, Melanogrammus aeglefinus and Arctogadus glacia-lis. Species substitution also involved the Asian sample SUR-13, where the declared Gadus cal-chogrammus was actually a mixture of species mostly belonging to Synodontidae, Clupeidae

and Lutjanidae families, as well as SUR-15 and SUR-16, where the reported Nemipterus spp.

was not found at all; (iv) 25% of the samples (all produced in non-EU countries) not reported

Fig 3. Genus presence and distribution in each sample. (E): SBP produced in Europe: (A): SBP produced in Asia.






at all the presence of molluscs on the label, despite mollusc DNA was found through our

molecular analyses (Fig 1).

Mislabelling and health implications

In parallel with the global increase of seafood consumption, seafood allergy incidence has con-

siderably risen over the past 40 years. To date, fish is considered one of the eight most common

allergenic foods, collectively considered to be responsible for about 90% of food allergic reac-

tions [24]. Symptomatology can be severe and sometimes fatal. Fish meat is in fact one of the

foods most commonly responsible of severe anaphylaxis [25]. Parvalbumin, which is found in

all fish species, is reported to be the major fish allergen for 95% of patients suffering from IgE-

mediated fish allergy [26]. It is resistant to boiling and other high temperature processing, so

that adverse reactions can also occur after consuming fish in processed form. Moreover, many

other allergens have been recently characterised and identified. Noteworthy, there is evidence

that the allergenic power of different fish species may differ to some extent, with e.g. hake and

cod reportedly being among the more allergenic [24]. Molluscs hypersensitivity also represents

a very common food allergy type. Consumption of molluscs is assumed to be responsible of

reactions ranging from a mild oral allergy syndrome to severe symptoms such as anaphylactic

shock in sensitive consumers. Tropomyosin (TM) was the first allergen identified, but also in

this case other allergens have recently been characterized [27]. Allergy to surimi has been veri-

fied in a patient who reacted to 1 g of surimi [28]. According to the current food European reg-

ulation, labels must inform consumers on the allergic hazard of all seafood types [9]. To make

a clearer and standardized labelling system, in terms of allergenic seafood, the terms “fish” and

Fig 4. Simpson’s diversity index (D Index) values obtained for each sample.






“molluscs” must be reported. This obligation is absolutely opportune since the presence of fish

and molluscs, even in small percentage, could represent a hazard in allergic consumers. In fact,

current clinical, epidemiological and experimental data do not allow determining safe allergen

threshold levels that would not trigger adverse reactions in a sensitised consumer [24]. How-

ever, deficiencies in food labelling, particularly in products imported from Asian countries,

have been often ascertained [29]. Therefore, the mislabelling rate involving the SBPs produced

in Asian countries further confirmed the lower safety degree of these products, above all con-

sidering the absence of a standardized system for seafood labelling and traceability [30]. In this

study, samples not reporting the presence of “fish” and/or “molluscs” on the label actually rep-

resent an health hazard for allergic consumers. Noteworthy, another possible health hazard is

represented by the presence of the species L. rivulatus in two Asian samples, which has been

reported to be associated to ciguatera poisoning (fishbase.org).

Species composition of SBPs and evaluation of environmental impact

If the main large-scale source for surimi production is the Alaska pollock (Gadus chalcogram-mus), the variability of fish stocks and the limitation of this species around the world has led to

Table 4. SBPs mislabelling cases evaluated through both labels information analysis and sequences data results.

Sample Label information (ingredients) Label conformity

Fish Molluscs

SUR-1 fish - cephalopod (molluscs);

- flavour (containing shellfish)

correct

SUR-2 surimi (fish) - cephalopod (mollusc):

- containing cephalopod ink

correct

SUR-3 surimi (fish) cephalopod (mollusc) correct

SUR-4 surimi [fish and cephalopods (molluscs)] correct

SUR-5 surimi (fish) extract (molluscs) correct

SUR-6 surimi (fish pulp Micromesistius poutassou) Flavorings

(contains shellfish)

mislabelled

B; C

SUR-7 surimi (fish) cephalopod (molluscs) correct


SUR-9 fish protein - cephalopod (molluscs);

- cephalopod ink (molluscs)

correct


SUR-11 surimi (fish) - cephalopod (molluscs);

- cephalopod ink (molluscs)

correct

SUR-12 white fish Squid mislabelled

B

SUR-13 Alaska pollack Gadus chalchogrammus NR mislabelled

A; C; D

SUR-14 surimi NR mislabelled

A; D

SUR-15 surimi Nemipterus spp. NR mislabelled

A; C(*); D

SUR-16 surimi Nemipterus spp. NR mislabelled

A; C(*); D

NR: not reported; A: not reporting the precise term “fish” among the ingredients; B: not reporting the precise term “molluscs” among the ingredients

(whereas declared); C: the voluntarily declared species not correspond to that retrieved by the analysis; D: label not declare the presence of molluscs but

the analysis proved the presence of species belonging to this Phylum;(*): the declared species is present but in lower amount respect to other undeclared species.






the exploitation of many other species, both among fish and cephalopods [2]. To date, more

than 80 species are in fact reported as a resource for surimi industry [21], belonging to a wide

and diverse taxonomic range. A recent study conducted by Galal-Khallaf et al. [5] has already

reported the use of a wide range of new species, sometimes vulnerable, in surimi production,

highlighting the necessity to proper identify species in such kind of foodstuff to better manage

overexploited and/or endangered marine resources. The results of our study further confirm

the presence of species traditionally used in surimi production, such as those belonging to the

groups of cods, haddocks and hakes. Alaska pollock (G. chalcogrammus), and in general spe-

cies included in the Gadus genus, was almost always found in both European and Asian SBPs.

Haddock (M. aeglefinus), already reported as commonly used, was in the same way often

found in European samples. On the contrary, to the best of our knowledge, whiting (M.mer-langus) and Arctic cod (A. glacialis) were not reported yet as a resource for surimi production.

As these two species were found in the samples produced in European countries, their pres-

ence could be explained by the fact that their habitat mostly includes the Northeast Atlantic

area, so that they are probably caught in European waters and then processed within the Com-

munity. The same occurs for the European hake (Merluccius merluccius), which has never

been reported in surimi production, while its congeners are worldwide exploited for this pur-

pose. On the contrary, Nemipterus spp., already reported as widely utilized in Asiatic products

[5], was found in the samples produced in the EU. Even though the identification at species

level was not reached, all Nemipteridae habit tropical and sub-tropical Indo-West Pacific

waters (fishbase.org). This implies that European surimi industry not only uses its own coun-

try resources, but also extra-EU species (which could be imported or directly caught in extra-

EU waters by European vessels). Moreover, since surimi can be processed from the flesh of

fish not only in land-based operations, but also on-board processing ships, it is not to consider

improbable that European ships operate in extra-EU waters, so that the products are conse-

quently composed by extra-EU resources.

Other species already reported in the literature were found in the analysed Asian samples,

such as Saurida undosquamis (Brushtooth lizardfish) and Trachurus spp. Noteworthy, the

presence of new unexpected species, generally characterized by low/not fishery interest, was

detected in both European and Asian samples. Among Clupeidae, Dorosoma petenense, a spe-

cies habiting North and Central America waters, and Ethmalosa fimbriata, from Eastern Cen-

tral Atlantic Ocean, were found in a sample produced in Korea, together with the Indo-Pacific

Coilia grayii (Engraulidae). Sander spp. (Percidae) was found in both European and Chinese

samples, but unfortunately the identification at species level was not reached, so that it was not

possible to attribute an origin to this fish. The Indo-Pacific Ptaerocaesio tile (Caesionidae) and

Siganus spp. (Siganidae) (which is commonly as used ornamental fish) were found in samples

produced in China and Thailand, respectively. Bengal snapper (Lutjanus bengalensis) and

Blupperlip snapper (Lutjanus rivulatus), habiting the Indian Ocean, were found in three Asian

samples. Finally, also freshwater fish were found: Etroplus maculatus, and Paretroplus macula-tus (Cichlidae), habiting Indian and African waters respectively, were found in a sample pro-

duced in Europe. P.maculatus, in particular, is reported as Critically Endangered (CE) species

by the IUCN Red List of Threatened Species (www.iucnredlist.org). Similarly, the freshwater

species Trichopodus leeri (Osphoronemidae), native of Malay Peninsula, Thailand and Indone-

sian waters, which was found in a Korean sample, is reported as Near Threatened (NT) by the

IUCN Red List of Threatened Species (www.iucnredlist.org). Among molluscs, whereas Dory-teuthis opalescens (Ommastrephidae) has already been reported for surimi production, to the

best of our knowledge no literature data concerning the use Todarodes pacificus (Ommastre-

phidae) and Architeuthis dux (Architeuthidae) have been reported yet.



http://www.iucnredlist.org

http://www.iucnredlist.org


Conclusions

In this work, the Ion Torrent NGS technology was tested for the first time on surimi-based

products. Although further studies aimed at optimizing and standardizing the analytical proto-

col, deepening the aspect of DNA quantification and obviously applying this technology to a

larger number of samples are required, the metabarcoding approach was proved to be suitable

for species identification of processed multispecies seafood products. Overall our results sug-

gest that surimi production is not related to a definite resource, but mostly depends to the

catching and processing area, fishing season and species availability. In particular, it seems that

in Asian region the overexploitation enforces the use of almost all of the catch for seafood

industry, often challenging the sustainable management and conservation of marine resources.

In fact, given the low percentage of DNA present in the SBPs for some species, it can be

excluded that these species are deliberately caught to be processed by the seafood industry but

rather they occasionally enter in the production chain. However, such a great and diversified

range of species, also including vulnerable and potentially toxic ones, as well as the often-unde-

clared presence of potentially allergic risks, strongly remarks the still too weak control system

and poorly eco-friendly management around the global seafood sector, highlighting the neces-

sity to further strengthen the tools aimed at ensuring both consumers and environmental

protection.

Acknowledgments

We thank Dr.ssa Lara Tinacci, FishLab, Department of Veterinary Sciences (University of

Pisa), for her help in collecting the samples.

Author Contributions

Conceptualization: Andrea Armani.

Data curation: Alice Giusti.

Formal analysis: Alice Giusti.

Funding acquisition: Carmen G. Sotelo.

Investigation: Alice Giusti.

Methodology: Carmen G. Sotelo.

Project administration: Carmen G. Sotelo.

Resources: Carmen G. Sotelo.

Software: Alice Giusti.

Supervision: Andrea Armani, Carmen G. Sotelo.

Writing – original draft: Alice Giusti, Andrea Armani.

Writing – review & editing: Andrea Armani, Carmen G. Sotelo.

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https://doi.org/10.1038/nrg2626

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